1. Field of the Invention
The present invention relates to an image processing apparatus capable of adjusting an output profile used for color matching in output devices.
2. Description of the Related Art
In color prints, adding spot colors (feature colors) such as red, green and blue to the normal four colors—three primary colors of cyan, magenta and yellow and black—can realize more vivid and natural color reproduction. For example, a color matching technique is known which converts a target value of the spot color into a CMYK value using an output profile, the table showing correspondence between Lab values of the Lab color space and CMYK values of the CMYK color space.
The color a printer produces changes depending on the compositions of ink and toner. Even if the same colors are to be produced, different printers require different methods. So, the conversion from Lab values to CMYK values is not an easy task. Under this circumstance, a conventional practice involves printing a color chart by a printer, measuring the color of the color chart with a spectrophotometer and, based on the result of measurement, adjusting an output profile. However, if the output profile has been adjusted properly, any change in the state of printer caused by subsequent environmental changes or other factors may result in a printed output of the printer deviating from the target Lab value. To cope with this problem, one method has been proposed to adjust the output profile when the output profile accuracy is bad or when a target Lab value cannot be obtained from the adjusted output profile because of the printer state variations (e.g., Japanese Patent Laid-Open No. 2005-167630).
However, in the technology disclosed in Japanese Patent Laid-Open No. 2005-167630, Lab colors of a plurality of patches that are shifted in a device-independent space (Lab space) depend on the accuracy of the output profile. So, there is no assurance that appropriate patch positions for interpolation estimation can be obtained. Since shifts are made in the Lab space, measured values outside or near the boundary of a printer gamut (color reproducing range of the printer) may get replaced with approximate device values depending on the output profile and thus cannot be enclosed by the interpolation color patches. There is another problem that color detection cannot be done outside the printer gamut.
For example, a color profile value for a device-independent spatial value (L0, a0, b0) is C0, M0, Y0, K0 and a color profile value for (L1, a1, b1) is C1, M1, Y1, K1. This means that device values of C, M, Y, K (4-dimensional spatial values) all assume different values. The color profile value is an output value obtained by converting a device-independent spatial value using color profiling.
If L0, a0, b0 and L1, a1, b1 are inside the printer reproducible color range and close to each other in terms of color difference, the L1, a1, b1 can be reproduced by shifting only three CMY device values—the CMY being fundamental colors of the printer subtractive color mixing—to C2, M2, Y2, K0 (K remains the same).
However, the real output profiling has some degrees of color reproduction accuracy problem, such as tone value loss and tone value inversion. For example, even if L0, a0, b0 of a device-independent space is close to L1, a1, b1 in terms of color difference, an actually reproduced color of C1, M1, Y1, K1, that are shifted four CMYK device values, not the three CMY device values, may differ greatly from that of C0, M0, Y0, K0. Another example is inversion phenomena in which a color value that is expected to lower the brightness (make the color dark) may instead result in a brighter reproduced color and in which a color value that is expected to make the color more vivid may result in a dull color.
As described above, when the profiled, reproduced color accuracy is bad or particularly when there are reproduced color ranges in which tone representation is bad, the relation among device-dependent spatial value groups that have been converted using this profile can differ greatly from the relation among the pre-conversion device-independent spatial value groups.
Even if color patch data is shifted in a device-independent color space for profile corrections to compensate for changes between two states that are close to each other in color difference such as those before and after a printer status change, conversion of the device-independent color patch data into the device-dependent space may fail to shift the reproduced color as expected.